1. Field of the Invention
The invention relates to a high frequency plasma apparatus, and more particularly to the high frequency plasma apparatus that utilizes the superposition of two standing waves, simultaneously generated but with different temporal and spatial patterns, so as to effectively improve the uniformity of plasma discharge.
2. Description of the Prior Art
Plasma-enhanced chemical vapor deposition (PECVD) has been widely applied to various hi-tech industries, such as semiconductor, flat display panel and solar cell industries. Currently, capacitive coupled plasma (CCP), typically operated at 13.56 MHz, remains the mainstream plasma source for PECVD. In order to further reduce the production costs by improving the throughputs, a higher deposition rate and a larger electrode are both necessary. However, due to the capacitive nature of CCP, the deposition of device-grade thin films at a higher rate cannot be achieved by simply raising the discharge power because a higher discharge power would result in more severe ion bombardment, which would lead to the deterioration of film quality.
One of the solutions to the problem mentioned above is to increase the applied frequency of PECVD, by which more electromagnetic power would be transferred to electrons and hence the fraction of electromagnetic power consumed by ions would be reduced. Consequently, the deposition rate can be improved substantially without compromising the film quality. Moreover, it needs to be mentioned that a higher frequency could improve the deposition rate more significantly. Nevertheless, the combination of higher frequency and larger electrode would lead to the problem of non-uniform plasma discharge caused by the standing wave effect.
Up to date, a number of techniques have been proposed to resolve the issue of non-uniform discharge induced by the standing wave effect. One of the existing techniques is to replace one of the parallel electrodes of conventional CCP with one having a concave space, which acts as a capacitor in series with the plasma region. The function of the concave space is to remove the excessive voltage higher than the lowest voltage over the powered electrode from the discharge region. However, the profile of the concave space must be designed according to the standing wave pattern that actually depends on a variety of parameters including the electrode dimensions, applied frequency, discharge gap and the number/locations of feed points. As a result, a new profile of the concave space is required if any of the parameters mentioned above is varied. More importantly, since this technique is based on the principle of moving the extra voltage higher than the lowest voltage over the powered electrode to the concave space, there is an upper bound for the frequency because the voltage will be entirely removed from the discharge region when the frequency is too high and a node is generate in the discharge region.
Another existing technique eliminates the standing wave effect by superposing two standing waves that are 90° out of phase in space and alternately generated in time. Nevertheless, this technique can only be operated in pulsed mode rather than in continuous-wave mode that is more commonly used for PECVD. In pulsed mode, the standing waves must be alternately generated in terms of time, such that unnecessary limitations for deposition processes might be imposed. For instance, for thin film silicon solar cell, it has been reported that pulsed mode results in not only a lower deposition rate of the silicon thin film, but also a poorer performance of conversion efficiency compared with continuous-wave mode.
As stated above, though a number of techniques in the art, such as the aforesaid resorts, can be applied to resolve the problem caused by the standing wave effect, yet the common shortcomings among these techniques still exist, which leads to the problem that they are only capable of generating uniform discharge in a narrow operation window.